This invention is concerned with the design of optical coatings for controlling the manner in which light of particular wavelengths is transmitted by or reflected from an optical surface.
Optical interference, which modifies the transmitted and reflected intensities of light, occurs with the superposition of two or more beams of light. The principle of superposition states that the resultant amplitude is the sum of the amplitudes of the individual beams. The brilliant colors, for example, which may be seen when light is reflected from a soap bubble or from a thin layer of oil floating on water are produced by interference effects between two trains of light waves. The light waves are reflected at opposite surfaces of the thin film of soap solution or oil.
One important practical application for interference effects in thin films involves the production of coated optical surfaces. If a film of a transparent substance is deposited on glass, for example, with a refractive index which is properly specified relative to the refractive index of the glass and with a thickness which is one fourth of a particular wavelength of light in the film, the reflection of that wavelength of light from the glass surface can be almost completely suppressed. The light which would otherwise be reflected is not absorbed by a nonreflecting film; rather, the energy in the incident light is redistributed so that a decrease in reflection is accompanied by a concomitant increase in the intensity of the light which is transmitted.
Considerable improvements have been achieved in the antireflective performance of such films by using a composite film having two or more superimposed layers. Two different materials may be used in fabricating such a composite film, one with a relatively high index of refraction and the other with a relatively low index of refraction. The two materials are alternately deposited to specified thicknesses to obtain the desired optical characteristics for the film. In theory, it is possible with this approach to design multiple layer interference coatings for a great variety of transmission and reflection spectrums. This has led to the development of many new optical devices making use of complex spectral filter structures. Antireflection coatings, laser dielectric mirrors, television camera edge filters, optical bandpass filters, and band rejection filters are some of the examples of useful devices employing thin film interference coatings.
One particular type of interference coating is the narrowband-pass filter, which is designed to allow wavelengths within a narrow range of the desired pass band to be transmitted, while a range of wavelengths on either side of the pass band are highly reflected. Such filters are known in the prior art with multiple pass bands, but have been limited by the requirement for the pass bands to be uniformly spaced in wave number. A more versatile design, in which the pass band wavelengths could be arbitrarily specified, would be useful in many applications.